Objectives Determine whether female marmosets undergo a reduction in bone mineral density following ovariectomy. Hypoestrogenism, whether naturally occurring (e.g. following menopause) or experimentally induced (e.g. by ovariectomy), has been found to cause a reduction in bone mineral density and increase in osteoporosis in all species in which it has been studied. In marmoset and tamarin monkeys, socially subordinate females regularly become anovulatory and may therefore remain hypoestrogenic for extended periods of time. To determine whether these animals might possess a mechanism to protect them from the deleterious skeletal consequences of hypoestrogenism, we compared bone-mineral density in ovary-intact and long-term ovariectomized common marmosets. Dual-energy X-ray absorptiometry was performed on seven nulliparous adult females undergoing ovarian cycles and on seven adult females that had been ovariectomized 12-21 months earlier. Bone mineral density was determined in the neck, chest, pelvis, arms, legs, abdomen, and spine, and percent body fat was determined in the chest and abdomen. Ovary-intact and ovariectomized animals showed no significant differences in any measures. These results contrast strikingly with those from other species and suggest that marmosets may possess a unique adaptation protecting them from hypoestrogen-induced reductions in bone mineral density. Thus, this species might provide a novel model for the investigation and treatment of osteoporosis. Key Words dual-energy x-ray absorptiometry, skeletal system, osteoporosis